Pressurized-water nuclear reactors comprise a vessel of generally cylindrical shape enclosing the reactor core, which vessel is disposed with its axis vertical in a cylindrical reactor pit having a lower bottom located vertically below the vessel. The nuclear reactor core is cooled by pressurized water circulating in the primary coolant circuit of the reactor and inside the vessel in contact with the fuel assemblies.
In the event of certain accidents arising in the nuclear reactor and resulting in a loss in the cooling function of the core, it is possible, though highly improbable, that serious consequences may ensue if the emergency injection circuits of the reactor cannot be put into operation. It is then possible for an accidental sequence to occur which leads to meltdown of the core in the absence of cooling water, which may involve destruction by piercing of the bottom of the vessel and flow of the mass of the molten core and of the materials surrounding the core into the concrete pit containing the reactor vessel. The contact of the molten mass of the fuel and the materials surrounding the fuel, called corium, the temperature of which may reach 2800.degree. to 3000.degree. C., with the bottom of the concrete reactor pit, in the absence of cooling, may lead to the complete destruction of the bottom of the pit. The corium may then force its way into the raft of the reactor containment shell, destroy this raft and contaminate the water table present in the ground of the nuclear-reactor site. The advance of the corium within the ground may only be stopped when the residual power of the corium has decreased sufficiently.
Various devices have been proposed for avoiding contact between the corium and the bottom of the concrete reactor pit.
The known devices generally enable the mass of corium to be spread out over a certain surface in order that the power to be removed per unit of surface is as low as possible and is compatible with the possibility of cooling by fluids. It has been proposed, for example, to recover and to contain the corium in a metal bag clad internally with refractory materials whose partial melting absorbs the energy, transiently, and provides a sufficient interval of time to externally immerse the metal bag in a mass of water, so as to remove the residual power of the corium by boiling of the mass of water.
The drawback of this device stems from the fact that the refractory materials are most often very poor heat conductors, which has the effect of increasing the equilibrium temperature of the corium which remains in the liquid state.
Other devices are known which use refractory hearths continuously cooled by a water circuit. One of the drawbacks of these devices is that the cooling circuit may have failures which are liable to render it at least partially ineffective. Furthermore, the heat exchange is not sufficiently great to prevent the corium from remaining at a high temperature and in the liquid state after its discharge onto the recovery and cooling device.
A device is also known which is constituted by a stack of sectional profiles placed horizontally in the bottom of the pit, beneath the bottom of the vessel, in such a manner as to constitute receptacles for the molten corium, so as to disperse the molten mass, to promote its cooling and to enable it to solidify. However, this device has the drawback of not effectively protecting the concrete of the reactor pit when the flow of the corium occurs in a localized manner. The sectional profiles which are disposed in a staggered fashion are then liable to be successively filled with molten corium by local overflow, such that the molten mass may rapidly reach the bottom of the reactor pit.